Method of modulating high frequency transmitters



Aug. 16, 1938. H. WE1-IRUN METHOD 0F MODULATING HIGH FREQUENCY TRANSMITTERS Filed Jan. 5, 1935 TV. w

Patented Aug. 16, 1938 2,127,148

UNITED STATES PATENT OFFICE METHOD F MODULATING HIGH FRE- QUENCY TRANSMITTERS Hans Wehrlin, Berlin-Lichterfelde, Germany, assignor to C. Lorenz Aktiengesellschaft, Berlin- Tempelhof, Germany Application January 3, 1935, Serial No. 283 In Germany January 6, 1934 3 Claims. (Ol. 179-171) There are known methods of modulation into a continuous current have been constructed wherein the amplitude of the carrier current is With a time constant of a denite relatively changed but wherein the mean value of the carsmall magnitude. The same constructions come rier current is maintained constant. It has been true in connection with the coupling elements v5v further proposed to change also the mean value between the modulation amplifier and theproper 5- of the carrier current on modulation in such modulation tube provided that the displacement a manner that it is low in case of small modulaof the Working point and the modulation of the tion Whereas it attains the mean value of the transmitter are effected in the same circuit of normal modulation in case of highest modulation the transmitter.

in order in this Way to effect savings of energy. Care must be taken that the change of modu- 10:

Such a change of the mean value of the carrier ldtiOIl and Of the WOIkiIlg DOrlii reSDeCtiVelY effe current may be effected by a displacement of the effected on the characteristic approximately at working point on the modulation characteristic. the Same time.

The displacement of the Working point is car- As the time constant is decisive for the trans- 1'5 ried cut by a direct current derived from the mission of the range of frequencies and as the 1:5.`

modulating current, which is displacing the transmission of the low frequencies becomes Working point t0 the lower limit in Case 0f Smau worse when the time constant is small, the ininodulation amplitudes and to the upper limit in Veniien DIODOSeS t0. 611110103 Coupling elemenS case of higher ones. having a time constant of a relatively small mag- In arrangements of this kind, however, it has fliilllde and i0 COIIlpeHSeile the dlOP Of 'Clie 10W 2.9 proved that the time constant for displacing the ffeelileleies Caused thereby in a DIeSiJege 0l the working point must be of a definite relatively modulation omplier by means of a Correspondsmail value in order to avoid a disadvantageous lng DlediStOItiOn. overmodulation that might occur in the first The effect of the invention is explained with moment of the modulation amplitude increasing. the eid O e fOlm 0f Construction SllOWll in lille ,2, 5 The displacement of the working point-as aldrawing. ready mentioned hereinbefore--is effected by a The 10W ffeqlleny mOdlll-eiiiOH iS intIOilCed continuous current derived from the modulating et l and takes WO different routes from there. frequencies. For smoothing the continuous cur- One 0f these, designated 2, extends through the rent there are required smoothingor so-called mcdlllation amplifier V, e usual 10W frequency 3,0

fiattening members- Those members comprise a amplier, to the terminal tube of this amplifier certain time constant. As the modulation is ap- Il Whilst the other I'Ollte (3) leads t0 the rec plied to the transmitter without delay, care must tie? 4 and OVel" il SmOOillllYlg deViCe A Mld a be taken that no diierences of time occur bereVelSng tlille 5 t0 i'Ille meddle/ting tube 9. Be-

tween the displacement of the Working point and tween the terminal tube 'l 0f the medulatioll 35 the arrival of the modulation in the transmitter. ampller 21nd the DTOPeI medulatiOn tube 9 there 1f the modulation wou1d reach the transmitter are disposed the coupling elements C and R. quiokor thon at any moment when the music The real displacement of the Working point is for instance Was just becoming louder an oVer effected in modulating choke 3 connected in the 40 modulation would happen if the continuous curgrid Circuit 0f the main Stage 5 0f the transmit- 40 ront oooting the displaoomont of the Working ter, which is controlled by the controlling transpotnt arrived later The Working point is then mitter or high frequency carrier source S. 'I'he not displaced whilst the modulation amplitudes modulation tube 9 inuences the modulating are already great enough. What is valid in re- Choke 8, disposed in the grid circuit of the main spect of the smoothing circuits is also valid in Stage 5. by means of which the change n mean 45 respect of the remaining circuits which comamplitude of the carrier is accomplished. By prise a time constant Whether they are disposed means 0f tube 9 110i'J Only iS the mOdllliting Signal in the modulation amplifier 01* the 50-Ca11ed C011.. dellVI'ed t0 the main transmitter but BJSO the pling circuits, i. e. in tlie circuits wherein the displacement of the Working point is effected by displacing device and the modulation devices are Changing the Centirluous Current flowing in one 50 coupled. Winding of the modulating choke corresponding In order to avoid the disadvantages of such to the change of the grid bias and thus of the an overmodulation the smoothing devices displate current of tube 9. The displacement of the posed behind the rectier by means oi which the Working point of the main stage depends upon rectied low frequency modulation is converted the grid bias of tube 9. 55

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The displacing of the working point is effected by the fact that the alteration of the grid bias of the tube 9, causes an alteration of the anode direct current, which in turn produces an alteration of the magnetization of the coil 8. The alteration of this direct current magnetization of the coil 8 causes a modification of the high frequency impedance of the circuit connected in advance of the transmitter 5, therefrom results the alteration of the amplitude of the carrier frequency transmitted from the control sender 5 to the Sender 5 so that it varies with the Value of the modulation amplitude.

The dimensioning of condenser C and resistance R which is decisive for the time constant according to the invention is so made that a small time constant may be obtained therefor which also inherently results in greater attenuation of the low frequencies. For balancing this drop of the amplitudes of the low frequencies a predistortion is carried out in one of the prestages of the modulation amplier V.

The time constant of the coupling elements C and R is added to the time constant essentially determined by the iilter device A. The time constant of these coupling elements is essentially given by the capacity C and the resistance combination Ram-'Flegrei RDMRl-Ral Rgkg Rak showing the constant anode-cathode-resistance of tube l and Rok@ being the grid-cathoderesistance of tube 9 dependent upon the variable working point.

For dimensioning the resistance R and the capacity C the lowest frequency to be transmitted is decisive besides the voltage-or power adaptation to the tube '1. The lower the frequencies to be transmitted the greater must be the product CRC, but on the other hand the time constant TZICERC shall be as small as possible.

The latter requirement can be accomplished only if, as mentioned heretofore, a drop of the amplitudes of low frequencies is admitted in these coupling elements and the balance therefor is effected at another point.

What is claimed is:

l. A modulating system comprising a high frequency carrier source, a low frequency signal source, means having a modulation characteristic, a multi-stage amplifier for amplifying the low frequency signal, means for causing the amplified low frequency signal to modulate the high frequency carrier at a point on said characteristic, means for displacing said point on said characteristic in accordance with the mean amplitude of the low frequency signal, means including a low time constant circuit coupling said second and third means to said modulating means, said low time constant causing a distortion of the low frequency signal and means for predistorting the low frequency signal in a complementary sense prior to the nal loW frequency amplification stage.

2. In a radio transmitter which comprises a device having a modulation characteristic, a coupling circuit having a time constant, means for applying a high frequency carrier to said device, a source of low frequency signals, a signal channel including said circuit for applying said signals to said device to effect modulation of said carrier by said signals at a point on said characteristic, means for producing control voltages in dependence on the mean value of said signals, and a control channel including part of said circuit for applying said voltages to said device to displace said point of effective modulation on said characteristic, the method of compensating for the distortion of the signals due to unequal drops` in amplitude thereof occasioned by the time constant of the said coupling circuit which comprises predistorting said signals in a complementary sense ahead of said coupling circuit.

3. A radio transmitter which comprises a device having a modulation characteristic, means for applying a high frequency carrier to said device, a source of low frequency signals, a rst channel for applying said signals to said device to effect modulation of said carrier by said signale, means for producing control voltages in dependence on the mean value of said signals, and a second channel partially coinciding with said iirst channel for applying said voltages to said device to control the mean amplitude of said carrier, a signal transmitting and amplifying circuit comprising a predistorting and amplifying device in said first channel, the portion of said rst channel which coincides with said second channel being proportioned so asto have a low time constant whereby frequency discrimination between different components of the signals results, and the predistorting and amplifying device being adapted to introduce an equal and opposite frequency discrimination.

HANS WEI-IRLIN. 

